Process and device for determination of a portion of a component of the air exhaled by a breathing living being characteristic of the metabolic function of such living being

ABSTRACT

The invention relates to a process for determination of a first component A a,1  in the air exhaled by a breathing living being characteristic of the metabolic function of such breathing living being, in particular the portion of carbon dioxide (CO 2 ) or oxygen (O 2 ), the exhaled air having at least two components (i=1, 2, . . . ) each having a portion A a,i , and the molecular weights M i  and the adiabatic coefficients k i  of the components being known, characterized in that the velocity of sound v s  in the exhaled air is measured, and in that the portion A a,1  of the first component in the exhaled air is calculated by use of the measured velocity of sound v s , the temperature T of the exhaled air, and the relation M G /k G =R×T/(V s   2 ), the statement M G =sum(A a,i ×M i ), i=1, 2, . . . , applying to the molecular weight M G  of the exhaled air, and the statement 1/(1−k G )=sum(A a,i /(1−k i )), i=1, 2, . . . , applying to the adiabatic coefficient k G , and R being the universal gas constant; the invention also relates to a device for application of this process.

[0001] The invention relates to a process for determination of the portion of the component of the air exhaled by a breathing living being characteristic of the metabolic function of the living being as specified in the preamble of claim 1 and to an appropriate device for application of this process.

[0002] The portion of carbon dioxide (CO₂) in exhaled air is an important measured variable as an alternative or in addition to electrocardiography or measurement of the oxygen saturation of the blood, both for diagnostic purposes and for continuous monitoring over a protracted period in the intensive care unit of a hospital or in the sleep laboratory. The processes disclosed in EP 0 309 666 A1, DE 39 36 825 C2, or U.S. Pat. No. 5,159,934 teach in this connection determination of the CO₂ portion of respiratory air by measurement of the absorption of infrared radiation.

[0003] In the so-called “mainstream process” the absorption section for the infrared radiation is mounted directly in the airway of the patient. This presents the disadvantage that the result of measurement may be falsified by precipitation of moisture onto the optical elements. In the so-called “sidestream process” a small part of the exhaled air is withdrawn and subjected to spectroscopic measurement remote from the patient. Aside from the danger of possible obstruction of the suction hose, this process presents the disadvantage that slight variations in the CO₂ concentration which may contain information of importance for diagnosis generally cannot be detected in practice by metrological means.

[0004] EP 0 653 919 B1 and CH 669 463 A5 disclose a process and/or a device for measurement of flow rate, volume of flow, temperature, and mean molecular weights of gases and mixtures of gases. To the extent that measurement of molecular weights is used for determination of various exhalation parameters in pulmonary function diagnosis, these documents teach use of a separate sensor for determination of the carbon dioxide or oxygen concentration.

[0005] Hence the invention is based on the problem of providing a process and/or an associated device which overcome(s) the disadvantages of the state of the art. In particular, determination of a component characteristic of the metabolic function is to be permanently reliable and possible at a high measurement rate and the associated device is to be cost-effective in production and in operation and rugged and easy to clean.

[0006] In addition, this device is to be of low weight and sterilizable with superheated steam.

[0007] The problem is solved by the process specified in claim 1 and the device specified in the coordinated claim. Particular embodiments of the invention are specified in the dependent claims.

[0008] The problem is solved by a process for determination of the portion A_(a,1) of a first component characteristic of the metabolic function of a breathing living being in the air exhaled by the living being, in particular the portion of carbon dioxide (CO₂) and/or or oxygen (O₂), the exhaled air having a minimum of two, but generally four, components (i=1, 2, . . . ), each having a portion A_(a,1), and the molecular weights M_(i) and the adiabatic coefficients k_(i) of the components being known, characterized in that the velocity of sound v_(s) is measured in the exhaled air and in that the portion A_(a,1) of the first component in the exhaled air is measured by use of the velocity of sound v_(s), the temperature T of the exhaled air, and the relation M_(G)/k_(G)=R×(T/v_(s) ²), M_(G)=sum of (A_(a,1)×M_(i)), i=1, 2, . . . applying to the molecular weight M_(G) of the exhaled air, 1/(1−k_(G))=sum of A_(a,1)/(1−k_(i)), i=1, 2, . . . to the adiabatic coefficient M_(G) of the exhaled air, and R is the universal gas constant.

[0009] This process affords among others the advantage that the adiabatic coefficient k_(G) for the gas mixture of the exhaled air neither need be precisely calculated nor determined with precision empirically. It rather suffices for the quotient of molecular weight M_(G) and adiabatic coefficient k_(G) to be determined at known temperature T in accordance with the relation indicated after metrological determination of the velocity of sound.

[0010] Except as otherwise indicated in what follows, the portions of the components always represent portions of volume by percent in relation to the total volume of air inhaled or exhaled. In principle both inhaled and exhaled air contain moisture. The moisture content is generally higher in the case of exhaled air than in that of inhaled air.

[0011] In principle, determination both of the portion of carbon dioxide and of the portion of oxygen as a characteristic component is to be considered. Since, however, the relative changes in portions between inhaled and exhaled air are generally greater in the case of carbon dioxide than in that of oxygen, in what follows preference will be given to consideration of this case, but without restricting the generality of the invention.

[0012] Under normal environmental conditions the portion A_(et,1) of the first component carbon dioxide (CO₂) amounts to 0.034% for dry air (moisture content=0). The portion A_(et,2) of the second component oxygen (O₂) typically amounts to 20.95%. The portion A_(et,3) of the third component, inert gases, nitrogen (N₂) in particular, typically amounts to 79.015%. These portions are naturally given and known. The fourth component is water vapor or moisture. In that the moisture content of inhaled air is calculated from the ambient temperature, the relative humidity, and the air pressure, determination may be made with the device claimed for the invention, by measurement of the velocity of sound, whether variations from this composition are present. The universal gas constant R amounts to 8.314 Ws/(K×mol).

[0013] The molecular weight M_(G) of the gas mixture of exhaled air is determined from the sum of the products of portions A_(a,i) of the individual components of the exhaled air and the molecular weights M_(i) of the individual components, which are known and may be taken from the appropriate technical literature. The adiabatic coefficient k_(G) of the gas mixture of exhaled air is to be determined by the portions A_(a,i) of the components of the exhaled air by means of the known adiabatic coefficients k_(i) of the components of the exhaled air, which are known and may be taken, for example, from the technical literature. Consequently, the equations indicated may be linked to each other in such a way that a specific solution for the portion A_(a,1) of the first component may be determined if it is assumed that the volume of the inert gas is of equal volume in the inhaled and the exhaled air and an estimate is adopted for the respiration quotient RQ.

[0014] In one particular embodiment of the invention the sum of the portion A_(a,1) of the first component and a portion A_(a,2) of a second component of the exhaled air more or less equals the sum of the portions A_(e,1) and A_(e,2) of the inhaled air, which are known or may be measured separately. In particular, the sums of the carbon dioxide and oxygen portions of the dry air portion of inhaled and exhaled air are more or less of equal value.

[0015] The respiration quotient RQ is defined as the volume of carbon dioxide volume given off relative to the volume of oxygen absorbed and according to data in the literature ranges from 0.7 or 0.8 to 1, depending, for example, on the state of nutrition of the patient. Consequently, the volume exhaled is smaller than the volume inhaled for a respiration quotient RQ smaller than 1.

[0016] Allowance for this circumstance is made as follows in the calculation algorithm. Let V_(a)+V_(e) be the average respiration volume standardized with respect to temperature and humidity, with V_(e) for the inhaled and V_(a) for the exhaled air volume. For example, let the respiration volume be standardized to a humidity of 0%, that is, dry air, and a predetermined temperature such as body temperature. The same applies correspondingly to the inert gas.

V _(e) ×A _(et,3) =V _(a) ×A _(at,3)

[0017] or, with a factor f introduced,

A _(at,3) =f×A _(et,3) or

f=A _(at,3) /A _(et,3) =V _(e) /V _(a)

[0018] Hence it is advantageous if, in an embodiment of the process claimed for the invention, the inert gas portion of the exhaled air is set to equal the inert gas portion of the inhaled air multiplied by the factor f.

[0019] The difference between inhaled and exhaled volume is described by the statement

V _(e) −V _(a)(A _(et,1) +A _(et,2) +A _(et,3))×V _(e)−(A _(at,1) +A _(at,2) +A _(at,3))×V _(a)

[0020] and if the portion A_(et,1) of the inhaled carbon dioxide in dry air is disregarded (A_(et,1)=0) and if allowance is made for the volume of the inert gas component not varying during respiration (A_(et,3)×V_(e)=A_(at,3)×V_(a)),

V _(e) −V _(a) =A _(et,2) ×V _(e)−(A _(at,1) −A _(at,2))×V _(a).

[0021] Together with the equation for the respiration quotient RQ, to which applies the statement

RQ=A _(at,1) ×V _(a)/(A _(et,2) ×V _(e) −A _(at,2) ×V _(a)),

[0022] with the portion A_(et,1) disregarded, there is obtained the relation

f=1+A _(at,1)×(1/RQ−1),

[0023] by means of which the ratio V_(e)/V_(a) may be estimated. When RQ=1, f=1, and when RQ=0.8, with A_(at,1). 0.06, f=1.02.

[0024] Adequate accuracy is attained for many applications if calculation is performed for factor f with a value corresponding to a respiration quotient RQ of 0.85. Should more precise measured values of the carbon dioxide concentration be required, inhaled and exhaled volume may be determined metrologically and factor f stated precisely.

[0025] The portion of a fourth component, moisture, generally present in exhaled air, may be assumed for many applications to be saturated water vapor. The portion of moisture is accordingly determined by the temperature T of the exhaled air and the barometric air pressure P_(bar) and thus may be derived from these values.

[0026] It may be sufficient for many applications to adopt the body temperature of the living being examined as the temperature T of the exhaled air. This body temperature may either be determined separately by metrological means, an empirically ascertained value may be used for which the specific measuring instrument setup is taken into account, or a conventional value may be adopted, such as a temperature of 37° C. in man.

[0027] If greater measurement accuracy is required, the temperature T of exhaled air may also be determined directly by metrological means. Numerous measurement processes, such as ones involving use of thermocouples or temperature-dependent resistors, are available in the state of the art for this purpose.

[0028] It may be advantageous for many applications, however, to determine the temperature T of the exhaled air metrologically with a dewpoint sensor. In this process the temperature of a bedewed surface is adjusted so that a still perceptible moisture precipitate is formed, one which may be determined in various ways, such as optically on the basis of the modified reflection or transmission pattern, or on the basis of change in an electric resistance, in electric capacitance, or in mechanical oscillation properties.

[0029] By preference use is made of an ultrasound signal, such as one of a frequency ranging from 50 to 200 kHz, especially around 120 kHz, for measurement of the velocity of sound v_(s). The velocity of sound is measured more or less at a right angle to the main direction of flow of the exhaled air. As a result, incidental effects are eliminated, for example, effects such as air turbulence which may occur during inhalation and exhalation. The measured value of the velocity of sound v_(s) may be verified by also measuring the velocity of sound of the inhaled air, the composition and accordingly the expected velocity of sound of which may be verified. The measured value of the velocity of sound v_(s) of the exhaled air is considered to be valid only if the value measured for the inhaled is in sufficiently close agreement with the expected value.

[0030] The problem formulated for the invention is also solved by a device for application of the process described in the foregoing, the device having a first measuring system for measurement of the velocity of sound v_(s) of the exhaled air, electronic storage means for storage of the molecular weights M_(i) and adiabatic coefficients k_(i) of the components of the respiratory air, and electronic computing means for calculation of the portion A_(a,1) of the first components. The electronic computing means and electronic storage means may preferably be configured in one module and, for example, as a plug-in card for a commercially available personal computer or for basic medical equipment already in existence. The values for the molecular weights M_(i) and/or the adiabatic coefficients k_(i) may be permanently set or programmable or predetermined.

[0031] In one particular embodiment of the invention the first measuring system with a first measurement axis is mounted in a respiration tube between a mouthpiece and an opening through which the exhaled air may be expelled. The measuring means of the first measuring system are preferably mounted on opposite sides of the respiration tube. The first measurement axis for measurement of the velocity of sound v_(s) preferably is oriented so that it forms a more or less right angle with the longitudinal axis of the respiration tube in the area of the first measuring system.

[0032] In one particular embodiment of the invention the device has a second measuring system with a second measurement axis in the respiration tube, one which is mounted so that the second measurement axis forms an acute angle with the longitudinal axis of the respiration tube in the area of the second measuring system. The first and second measurement axes preferably form a more or less right angle. The second measuring system may be used in particular for metrological determination of the volume of the inhaled air and/or the exhaled air and accordingly of the ratio f=V_(e)/V_(a). In addition, the second measuring system may be used to determine other parameters of the respiratory air, such as the volume flow or the rate of flow.

[0033] In one particular embodiment the temperature of the exhaled and/or inhaled air is measured by means of a sensor. Use is preferably made for this purpose of a dewpoint sensor the temperature of which is set so that the moisture in the gas surrounding the sensor begins to condense. If desired, both a dewpoint sensor and a temperature sensor may be used.

[0034] Other advantages, features, and details of the invention may be determined from the dependent claims and the following description, in which a plurality of exemplary embodiments are described in detail with reference to the drawing. The features specified in the claims and in the description may be essential to the invention individually or in any combination.

[0035]FIG. 1 presents a side view of a cross-section of a first exemplary embodiment of the device claimed for the invention,

[0036]FIG. 2 a front view along line II toward the device shown in FIG. 1,

[0037]FIG. 3 a top view of a cross-section of a second exemplary embodiment of a device claimed for the invention, and

[0038]FIG. 4 a side view of a cross-section of a third exemplary embodiment of a device claimed for the invention.

[0039] An exemplary embodiment of the process claimed for the invention is described in what follows with reference to an independently breathing patient. In this case it is to be stipulated that the composition of the inhaled air is known. If the portions of the inert components of the air, those of nitrogen and other inert gases in particular, are combined in one inert gas mixture which does not undergo chemical change during the process of respiration, the portions of the inhaled, dry air in terms of volume in relation to the total volume of the inhaled air may be stated as follows: carbon dioxide: A_(et,1) = 0.034% oxygen: A_(et,2) = 20.950% inert gases: A_(et,3) = 79.015% moisture: A_(et,4) = 0%

[0040] The following are obtained for the portions of exhaled air by volume for the present exemplary embodiment:

[0041] The body temperature of the patient is adopted for the temperature T of the exhaled air. Saturation is assumed for the moisture content of the exhaled air. Consequently, the saturation pressure P_(s) of the moisture or of the water vapor contained in the exhaled air depends exclusively on the temperature T and may be calculated by means of the so-called Magnus formula, for example:

P _(s) in millibars=6.112×exp(17.62×T/(243.12+T))  (1)

[0042] The known or measured barometric air pressure P_(bar) is used to calculate the partial pressure and accordingly the portion by volume

A _(a,4) ═P _(x) /P _(bar)  (2)

[0043] of the water vapor or the moisture as a function of the temperature T. The portion by volume of the inert gas is

A _(a,3) =f×A _(et,3)×(1−Aa ₄)  (3)

[0044] The portion A_(a,1) of the carbon dioxide by volume is the quantity sought. The portion A_(a,2) of the oxygen may be stated as

A _(a,2)=1−A _(a,1) −A _(a,3) −A _(a,4)  (4)

[0045] The following equation

v _(s)={square root}(k _(G) ×R×T/M _(G))  (5)

[0046] in which k_(G) is the adiabatic coefficient of the gas mixture, R is the universal gas constant, and M_(G) the molecular weight of the gas mixture, is valid for the measured velocity of sound v_(s). The quotient of molecular weight M_(G) and adiabatic coefficient k_(G) as a function of temperature T and the measured velocity of sound v_(s) may be expressed by transposition of this equation:

M _(G) /k _(G) =R×T/(v _(s) ²)  (6)

[0047] The portion A_(a,1) of the carbon dioxide to be determined may then be determined for each individual measured value of the velocity of sound v_(s), the following statement applying to the molecular weight M_(G):

M _(G)=sum of all values i(A _(a,i) ×M _(i)), with i=1, 2, 3, 4  (7)

[0048] while the following applies to the adiabatic coefficient k_(G):

1(1−k _(G))=sum of all values i(A _(a,i)/(1−k _(i))) with i=1, 2, 3, 4  (8)

[0049] Equations (1) to (8) make up a system of equations with an unequivocal solution for the portion A_(a,1) of the carbon dioxide as a function of the velocity of sound v_(s) and temperature T. Consequently, it is necessary to know only the velocity of sound v_(s), the barometric air pressure P_(bar), and the temperature T of the air in the respiratory passage with the highest possible accuracy. The calculation may be performed with commercially available computing means, for example, also with a personal computer which executes an appropriate measuring and/or computing program.

[0050]FIG. 1 presents a side view of a cross-section of a first exemplary embodiment of the device 1 claimed for the invention for determination of a portion of a component of the air exhaled by a living being characteristic of the metabolic function of the breathing living being, in particular for determination of the portion of carbon dioxide in the air exhaled by a human patient.

[0051] In the first exemplary embodiment, a first measuring system 3 is mounted in a more or less cylindrical respiration tube 2 for determination of the velocity of sound v_(s) of the respiratory air flowing through the respiration tube 2. The principal direction of flow of inhaled air is indicated by the arrow 4 and that of exhaled air by the arrow 5. The directions of flow extend more or less parallel to the longitudinal axis of the respiration tube 2. On opposite sides relative to the longitudinal axis 6 the first measuring system 3 has an ultrasonic sensor 7 and an associated receiver 8. The transmitter 7 transmits a signal of predetermined signal shape, the reception of which signal is registered at the receiver 8, and the velocity of sound v_(s) of the respiratory air is determined from the transit time and the known distance between transmitter 7 and receiver 8. The first measurement axis 9 forms a right angle with the longitudinal axis 6 of the respiration tube 2.

[0052] On one of its ends the respiration tube 2 has a mouthpiece 10 which is integrated with the tube or is removable, especially for cleaning purposes. The opposite end of the respiration tube 2 is open and respiratory air may flow in or out freely through the associated opening.

[0053] A sensor element 11 is mounted in the respiration tube 2, in the immediate vicinity of the first measuring system 3 or even connected to it especially by electric means. The sensor element 11 has a dewpoint sensor 13 with a temperature sensor 14 on an electrically heatable substrate 12. The substrate 12 is heated to a temperature such that a still detectable moisture precipitate is formed on the dewpoint sensor 13. The temperature sensor 14 measures the pertinent temperature from which the portion of moisture in the respiratory air is in turn derived in accordance with the Magnus formula referred to above.

[0054] The electric signals are delivered over a connecting line 15 to an evaluation unit 16, possibly also remote from the patient examined. This evaluation unit 16 has electronic storage means 17 in which are stored the coefficients and constants required for calculation, ones which are freely programmable and thus may be updated. In addition, the evaluation unit 16 also has electronic computing means 18 which presents the result determined on a display unit 19 and/or at a connecting element 20 for forwarding to a downstream unit.

[0055]FIG. 2 presents a front view along line II to the device shown in FIG. 1 in the direction of the longitudinal axis 6. The more or less cylindrical cavity in the respiration tube 2 is interrupted only in the area approximately central in the axial direction by the transmitter 7 and receiver 8, which preferably are mounted plane-parallel relative to each other.

[0056]FIG. 3 presents a top view of a cross-section of a second embodiment 101 of a device claimed for the invention. The first measuring system (not shown in detail in FIG. 3) has the first measurement axis 109 extending perpendicular to the plane of the drawing. As has been specified in the foregoing, the portion A_(a,1) of the first component in question of the gas mixture of the respiratory air is thereby determined.

[0057] The volume of inhaled and exhaled air is determined by a conventional method by a second measuring system 121 whose measurement axis 122 forms with the longitudinal axis 106 of the respiration tube 102 an acute angle 45°, for example. On the basis of the numerical values involved obtained for the respiration quotient RQ, which also depends among other things on the nutritional habits of the patient examined and may range from 0.7 to 1, the ratio f of inhaled to exhaled volume of the air standardized with respect to temperature and humidity lies between 1 and 1.025. A fixed value of 1.01, for example, which yields adequate measurement accuracy for many applications, may be adopted for factor f in place of the ratio which may be determined with precision by the second measuring system 121. The signals of the first and second measuring systems 3, 121, as well as those of any sensor elements present 7, may be transmitted over a common connecting line 115.

[0058] In the second exemplary embodiment shown, the first and second measurement axes 109, 122 intersect. It may be preferable for many applications to mount the first measurement axis 109 offset a certain distance, especially in the direction of the longitudinal axis 106, from the second measurement axis 122. By preference the offset of the first measurement axis 109 toward the mouthpiece of the respiration tube 102 amounts, for example, to 1 to 5 cm, in particular 2 to 3 cm, as is indicated by a broken line in FIG. 3 for the first measuring system 103′. As a result, interference of the measuring process of the first and second measuring systems 103′, 121, for example, may be prevented.

[0059]FIG. 4 presents a side view of a cross-section of a third exemplary embodiment 201 of a device claimed for the invention. The device 201 differs from the first embodiment shown in FIG. 1 among other things in that the dewpoint sensor 213 is mounted between the first measuring system 203 and the mouthpiece 210 of the respiration tube 202. The dewpoint sensor 213 is heated by an annular heating sleeve 223. The moisture precipitate on a reflecting precipitate surface 224 is detected by evaluation of the optical reflection pattern. For this purpose an optotransmitter 225, such as a light emitting diode, directs a light beam 227 at an acute angle to the precipitate surface 224. Optimal reflection by the optoreceiver 226, such as a photodiode, is obtained if no moisture precipitate is present on the precipitate surface 224.

[0060] The temperature of the precipitate surface 224 is adjusted so that a barely sufficient precipitate is formed which can still be detected with certainty by the optoreceiver 226. The reflecting effect of the precipitate surface 224 may be obtained, for example, by appropriate metal coating of the surface. The configuration of the precipitate surface 224 may be oblong or also more or less punctate; in any event it is preferably positioned between the optotransmission element 225 and the optoreceiver 226. In order for the precipitate surface 224 to be positioned to the greatest extent possible in the direct airstream of the respiratory air in the respiration tube 202, the optotransmitter 225 and the optoreceiver 226 may be mounted so as to be offset from each other relative to the longitudinal axis 206, that is, they are not aligned with each other as the device 201 is viewed from the top, in the direction of the longitudinal axis 206 but rather are offset a certain distance in the circumferential direction.

[0061] As an alternative to the optically operating dewpoint sensor shown, a moisture precipitate may also be detected by evaluating the change in an electric resistor, an electric capacitance, or the mechanical oscillatory pattern. The temperature at which a still measurable moisture precipitate occurs is employed as the temperature T of the respiratory air. If necessary, allowance may be made for a correction factor or correction value, one obtained by empirical means, for example. 

1. A process for determination of a portion A_(a,1) of a first component in the air exhaled by a breathing living being characteristic of the metabolic function of such breathing living being, in particular the portion of carbon dioxide (CO₂) or oxygen (O₂), the exhaled air having at least two components (i=1, 2, . . . ) each having a portion A_(a,i), and the molecular weights M_(i) and the adiabatic coefficients k_(i) of the components being known, characterized in that the velocity of sound v_(s) in the exhaled air is measured, and in that the portion A_(a,1), of the first component in the exhaled air is calculated by use of the measured velocity of sound v_(s), the temperature T of the exhaled air, and the relation M _(G) /k _(G) =R×T/(v _(s) ²), the statement M _(G)=sum(A _(a,i) ×M _(i)), i=1, 2, . . . applying to the molecular weight M_(G) of the exhaled air, and 1/(1−k _(G))=sum(A _(a,i)/(1−k _(i))), i=1, 2, . . . applying to the adiabatic coefficient k_(G) of the exhaled air, and R being the universal gas constant.
 2. The process as claimed in claim 1, wherein, in calculation of the portion A_(a,1) of the first component of the exhaled air, it is considered that the sum of the portion A_(a,1) of the first component and a portion A_(a,2) of a second component of the exhaled air more or less equals the sum of the portions A_(e,1), A_(e,2) of these components of inhaled air, which are known or are measured.
 3. The process as claimed in claim 1 or 2, wherein, in calculation of the portion A, of the first component of the exhaled air, it is taken into consideration that a third component of the inhaled and exhaled air is inert.
 4. The process as claimed in one of claims 1 to 3, wherein, in calculation of the portion A_(a,1) of the first component of the exhaled air, one portion A_(a,4) of moisture contained in the exhaled air is assumed to be saturated water vapor, and wherein the portion A_(a,4) of the moisture is derived from the temperature T of the exhaled air.
 5. The process as claimed in one of claims 1 to 4, wherein the temperature T of the exhaled air is measured.
 6. The process as claimed in claim 5, wherein the temperature T of the exhaled air is measured with a dewpoint sensor.
 7. The process as claimed in one of claims 1 to 6, wherein the velocity of sound v_(s) is measured with an ultrasonic signal.
 8. The process as claimed in one of claims 1 to 7, wherein the velocity of sound v_(s) is measured more or less at a right angle to the direction of principal flow of the exhaled air.
 9. The process as claimed in one of claims 1 to 8, wherein, for the purpose of verification of the measured value of the velocity of sound v_(s) of the exhaled air, the velocity of sound of the inhaled air is also measured and compared with an predetermined nominal value.
 10. A device (1; 101; 201) for application of the process as claimed in one of claims 1 to 9 for determination of a portion A_(a,1) of a first component, characteristic of the metabolic function of a breathing living being, in the air exhaled by the living being, in particular the portion of carbon dioxide (CO₂) or of oxygen (O₂), characterized in that the device (1; 101; 201) has a first measuring system (3; 203) for measurement of the velocity of sound v_(s) of the exhaled air, electronic storage means (17) for storage of the molecular weights M_(i) and the adiabatic coefficients k_(i) of the components, and electronic computing means (18) for calculation of the portion A_(a,1) of the first component.
 11. The device (1; 101; 201) as claimed in claim 10, wherein the first measuring system (3; 203) is mounted with a first measurement axis (9; 109) in a respiration tube (2; 102; 202) between a mouthpiece (10; 210) and an opening by way of which the exhaled air may be discharged, wherein means of the first measuring system (3; 203) are mounted on opposite sides of the respiration tube (2; 102; 202), and wherein the first measurement axis (9; 109) for measurement of the velocity of sound v_(s) forms with the longitudinal axis (6; 106; 206) of the respiration tube (2; 102; 202) a more or less right angle in the area of the first measuring system (3; 203).
 12. The device (1; 101; 201) as claimed in claim 11, wherein the second measuring system (121) is mounted with a second measurement axis (122) in the respiration tube (2; 102; 202), wherein the second measurement axis (122) forms an acute angle with the longitudinal axis (6; 106; 206) of the respiration tube (2; 102; 202) in the area of the second measuring system (121), and wherein the first and the second measurement axes (109; 122) form a more or less right angle.
 13. The device (1; 101; 201) as claimed in one of claims 10 to 12, wherein the temperature of the exhaled and/or the inhaled air may be measured by means of a sensor, in particular a dewpoint sensor (13; 213). 